The present invention relates to a method and apparatus for ionization detection useful in gas chromatography (GC). More specifically, the invention provides for a variable pressure ionization detector, in particular an electron capture detector in a variable pressure vacuum chamber, which allows selectivity in the detection of gaseous organic compounds by variation of the pressure and/or chemical composition of the ionization reagent gas in the detector.
Radiation ionization cells have heretofore been used extensively for gas chromatographic detection. The basic design typically consists of a two electrode, low volume cell which houses a small quantity of a beta particle emitter such as .sup.3 H or .sup.63 Ni. Interaction of the beta radiation with the reagent gas, GC carrier gas, and effluent generates a population of free electric field across the electrodes. Depending on the nature of the carrier and/or reagent gas used, as well as the geometry of the cell, the measured electron current will either increase (due to electron emission) or decrease (due to electron capture) as compounds elute from the GC column and enter the detector cell. This response is amplified and recorded to generate a chromatogram.
Numerous variations of this basic cell design have been disclosed, some of which have selective response and others of which are essentially universal detectors. The more important of these include the electron capture detector (ECD), the argon ionization detector, the micro argon detector, the helium ionization detector, and the cross-section ionization detector. With the exception of the ECD, these detectors respond by means of various reaction mechanisms which cause electron emission (increased current) as eluted compounds traverse the cell. Response is also affected by the manner in which free electrons are collected. For example, the electric field applied to the cell may be either continuous (DC) or pulsed. Wider dynamic range, greater stability, and fewer aberrations are typical characteristics of the pulsed systems.
In cross-section ionization detectors, compounds are directly ionized by collision with beta particles and high energy electrons. The measured current of ejected electrons is a function of compound concentration and ionization cross-section.
Both the argon and helium ionization detectors generate a response through ionizing collisions of metastable noble gas atoms with the GC effluent. By using high purity argon or helium as the carrier gas and/or by adding these gases to the cell as reagents, a high concentration of metastable atoms is produced by the beta radiation. Since the metastable states of these atoms are above the ionization potentials of most compounds, ionizing collisions occur as eluted compounds enter the detector. Again, the increase in free electrons is measured and recorded as a response. This is a more efficient process than cross-section ionization, and therefore the sensitivity is higher.
Electron capture detectors in general have been used in gas chromatography as disclosed in "Gas Chromatography Principles, Techniques and Applications," Littlewood, A. B., Electron Capture Detectors, 2nd Ed., Academic Press, N.Y., (1970), pp. 315-322. The electron capture cell contains a small quantity of .sup.63 Ni foil which emits beta particles. Under conventional operating conditions, i.e., atmospheric pressure, the beta particles collide with atoms and molecules in the GC carrier gas, usually argon with 10% methane, resulting in a large population of free electrons formed by collisions of methane with metastable argon atoms. Ejected electrons are rapidly thermalized through collisions with neutral methane and argon. Steady state currents (standing currents) of 10.sup.-8 to 10.sup.-9 amps are produced. As compounds elute from the GC column and enter the detector cell, they capture electrons (provided that they have a high enough electron affinity) and cause a decrease in the measured electrical current. This change in current is measured and recorded to produce a gas chromatogram.
Negative ion chemical ionization mass spectrometry has been shown to be a useful method for discriminating, for example, between isomeric polycyclic aromatic hydrocarbons (PAH). The article "Differentiation of Polycyclic Aromatic Hydrocarbons Using Electron Capture Negative Chemical Ionization", Buchanan, M. V. and Olerich, G., Org. Mass. Spec. Vol. 19, No. 10, 1984 describes the use of electron capture ionization of PAH compounds to produce molecular ions, [M]-. Differentiation of isomeric PAH compounds could be effected based on relative differences in electron affinity. These experiments also indicated that the degree of discrimination was pressure dependent and increased at lower pressures. Because of the similar nature of the processes that occur in a conventional electron capture detector (ECD) used for gas chromatography it was believed that similar isomeric discrimination could be achieved with a GC by operating an ECD at reduced pressures.
The known prior art arrangements have always been operated at atmospheric pressure, so that a fixed number of low energy electrons (for a given carrier gas) are available for electron capture. A threshold electron affinity is thus established, above which electrons are captured and below which electrons are not captured. This phenomenon can be used to advantage, for example, in differentiating two isomers, one of which has an electron affinity above the threshold and the other below. If both isomers are above the threshold, however, such differentiation is not possible, since both compounds will affect the measured electrical current. The prior art electron capture detectors are thus limited in their usefulness for differentiating compounds.